HEMT (high electron mobility transistor) is one example of traditional power semiconductor devices. HEMT is manufactured using a III group nitride semiconductor, which means semiconductor alloyed metal from GaN, AlGaN, InGaN or AlInGaN system, as mentioned in this specification.
As known in the traditional art, HEMT includes a first III group nitride semiconductor body composed of, for example, undoped GaN, and a second III group semiconductor body which is arranged on the first III group nitride semiconductor body and is composed of, for example, AlGaN.
As well known in the art, hetero-junction between the first III group nitride semiconductor body and the second III group nitride semiconductor body generally forms a conductive region which is also referred to as “two-dimensional electron gas (2DEG).” Typical HEMT further includes at least two power electrodes. Current flows between the two power electrodes through the 2DEG.
HEMT further includes a gate arrangement which is operable to activate or deactivate the 2DEG as desired, thereby turning on or off the HEMT. As a result, HEMT may act as a field effect transistor (FET). In fact, this HEMT is also referred to as “hetero-junction field effect transistor (HFET).”
III group nitride-based hetero-junction power semiconductor devices having high current transporting capability and high breakdown voltage capability are adaptable for power applications because of their low power loss. However, a III group nitride semiconductor device is generally of a normally on type, which means that a bias is required to be applied to a gate electrode to turn off this device.
Normally on devices are less preferable for power applications, because (a) these devices are operated with less efficiency than normally off devices and (b) driving circuits for the normally on devices are complicated and expensive. Thus, there is a need to provide a normally off III group nitride power semiconductor device.
An AlGaN/GaN hetero-junction structure is used for a high power field effect transistor which controls a flow of source-drain current through a gate voltage using 2DEG at an AlGaN/GaN interface. This 2DEG is generated according to a polarization effect by which negative charges are generated below an AlGaN surface. These negative charges are very sensitive to the surrounding environments, which results in fluctuation of the source-drain current.